The following description relates to one or more scanning optical systems configured to be incorporated in an image forming device.
A conventional scanning optical system has generally been configured as follows. Laser light emitted by a light source is converted into parallel light through a collimating lens, and thereafter made incident onto a deflector. The light deflected by the deflector is scanned in a predetermined direction on a scanned surface at a constant speed maintained via an fθ lens group. The conventional scanning optical system has usually been configured with a polygon mirror employed as a deflector. In recent years, a scanning optical system is required to be miniaturized to meet a requirement for downsizing an image forming device. Thus, as exemplified in Japanese Patent Provisional Publication No. HEI 10-232347 (hereinafter referred to as '347 Publication), there has recently been proposed a scanning optical system which can be miniaturized.
It is noted that a direction in which the laser light deflected by the deflector is scanned on the scanned surface will be defined as a main scanning direction in the following description. Additionally, a direction perpendicular to an optical axis of the scanning optical system and the main scanning direction will be defined as an auxiliary scanning direction. Each direction used when describing elements other than the scanned surface will be determined based on the directions defined on the scanned surface.
The scanning optical system disclosed in '347 Publication is provided with a converging lens as substitute for the collimating lens so as to convert the laser light emitted by the light source not into parallel light but into light converged in the main scanning direction. Thereby, a back focus of the scanning optical system is shortened so as to maintain a focal length of the scanning optical system and to hardly reduce a scanning amount with respect to a rotation angle of the polygon mirror. Consequently, a field angle required for the fθ lens group provided between the polygon mirror and scanned surface is constrained, and therefore the scanning optical system can be miniaturized, keeping favorable optical performances.
In case of using the polygon mirror as the deflector, there is known such a phenomenon that an incident point, namely, a reflection point of the laser light on the polygon mirror (hereinafter referred to as a deflection point) is slightly shifted during a single scanning operation. The positional shift of the deflection point is caused due to an effect of the polygon mirror in which plane deflection surfaces are rotated, with a value up to a radius difference between an inscribed circle and a circumscribed circle of the polygon mirror. When using the converged light, the shift of the deflection point causes an asymmetric scanning trajectory with respect to the optical axis of the fθ lens group, or more specifically, an asymmetric aberration (note: the scanning trajectory represents a trajectory of an imaging point of the converged light deflected by the polygon mirror, which imaging point is defined under the assumption that the converged light is not transmitted through the fθ lens group, as shown in FIG. 6). Therefore, the scanning optical system disclosed in '347 Publication employs the fθ lens group designed such that power allocation thereof in the main scanning direction is asymmetric with respect to the optical axis thereof, in order to overcome the aforementioned asymmetric aberration.
However, when using the fθ lens group with the asymmetric power allocation disclosed in '347 Publication, an allowable margin of location error of each optical element such as the fθ lens group and coupling lens group provided between the light source and polygon mirror is narrower. In other words, the optical performances of the entire scanning optical system are very sensitive to a positional error of each optical element dislocation. Namely, a minute positional error caused when each element is attached results in unacceptable level of astigmatism and shift of the scanned position on the scanned surface.